1. Field of the Invention
The present invention relates to a coating material comprising a saponification product of modified ethylene-vinyl acetate copolymer. The coating material affords a coating film which increases in strength upon heating. The copolymer contains as a third component olefinically unsaturated monomers capable of cross-linking. The saponification product is readily soluble in solvents and has an intrinsic viscosity (abbreviated as [.eta.] hereinafter) not greater than 0.7 dl/g which is measured at 20.degree. C. using phenol containing 15 wt% of water as a solvent.
2. Description of the Prior Art
The saponification product of ethylene-vinyl acetate copolymer (abbreviated as EVOH hereinafter) is a useful polymeric material superior in gas barrier properties, clarity, oil resistance, and keeping of fragrance. On account of these characteristic properties, it finds broad applications as film, sheet, and containers. It is also used in the form of solution to coat the surface of film, sheet, or containers made of a variety of resins. The coating of EVOH improves the film, sheet, and containers in oil resistance and gas barrier properties. In order for the coating of EVOH to have sufficient strength, EVOH should have a certain degree of polymerization, or have an intrinsic viscosity greater than 0.7 dl/g, preferably greater than 0.9 dl/g. On the other hand, EVOH having such a high intrinsic viscosity yields highly viscous solutions when dissolved in a solvent in high concentrations even though the coating solution of high concentration is economically advantageous. Thus the resin concentration of the coating solution is limited to about 15% at the highest from the standpoint of workability. This limited concentration makes it necessary to repeat the coating operation until a desired coating thickness is obtained, and the repeated coating is uneconomic from the standpoint of solvent recovery.
Japanese Patent Laid-open No. 123189/1975 (counterpart of U.S. Pat. No. 4097436) discloses a process for producing silicon-containing polyvinyl alcohol by copolymerizing vinyl acetate with vinyl alkoxysilane and subsequently saponifying the resulting copolymer. It also discloses that a portion (30 to 40 wt%) of the vinyl acetate used for the copolymerization can be replaced by other comonomer (such as ethylene). It further discloses that an aqueous solution of the modified polyvinyl alcohol can be used as a coating material for inorganic silicate substrates. However, it gives no description about silicon-modified EVOH containing 25 to 55 mol% of ethylene and the use of this polymer as a coating material in the form of solution in a mixed solvent of water and organic solvent.
Japanese Patent Laid-open No. 76403/1983 discloses a process for producing a modified polyvinyl alcohol having cross-linkable groups by copolymerizing vinyl acetate with N-alkoxymethyl(meth)acrylamide. It also discloses that the copolymerization is possible for other comonomer such as ethylene in an amount less than 10 mol%. However, it gives no description about EVOH containing 25 to 55 mol% of ethylene and having cross-linkable groups.
Japanese Patent Publication No. 48489/1972 discloses the use of EVOH as a coating material in the form of solution in a mixed solvent of water and organic solvent (such as n-propyl alcohol). However, it gives no description about the modification of EVOH with cross-linkable monomers and the low-molecular weight EVOH having an intrinsic viscosity lower than 0.7 dl/g.